Dinuclear iron(0) complexes of N-heterocyclic carbenes

Article abstract of DOI:10.1021/om401039z

The synthesis, structures, and reactivity of dinuclear Fe⁰ complexes of N-heterocyclic carbenes (NHCs, denoted as L) are reported. The NHC adducts of ferric chloride (L)FeCl₃ were prepared from the reactions of FeCl₃ with

Full text of DOI:10.1021/om401039z

Article
pubs.acs.org/Organometallics
Dinuclear Iron(0) Complexes of N‑Heterocyclic Carbenes
Takayoshi Hashimoto,† Ryoko Hoshino, Tsubasa Hatanaka,‡ Yasuhiro Ohki,* and Kazuyuki Tatsumi*
Department of Chemistry, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: The synthesis, structures, and reactivity of
dinuclear Fe0 complexes of N-heterocyclic carbenes (NHCs,
denoted as L) are reported. The NHC adducts of ferric
chloride (L)FeCl3 were prepared from the reactions of FeCl3
with L in toluene. The reduction of (L)FeCl3 with KC8
resulted in the formation of the dinuclear Fe0 complexes
Fe2{μ-η1(C):η6(arene)-L}2 (2a, L = 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene (IMes); 2b, L = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr)), in which
NHC ligands bridge two iron atoms using one of the arene
rings as an η6 ligand. Their magnetic properties are different: 2a is paramagnetic and 2b is diamagnetic. The dinuclear complexes
2a,b serve as precursors for monomeric (NHC)Fe0 species, and treatment of 2a,b with 1 atm of CO led to the formation of
(L)Fe(CO)4. Complex 2a was found to react with 1-azidoadamantane, giving rise to the dinuclear tetrazene complex
(IMes)Fe(μ-NAd)2Fe(AdNNNNAd) (4).
include Fe2−carbonyl−dithiolate complexes of NHC ligands
mimicking the active site of [FeFe]-hydrogenase,11 NHC
adducts of Fe(COT)2 (COT = cyclooctatetraene),12 FeI and
Fe0 complexes having a tridentate or a tetradentate ligand
containing NHC groups,13 and carbonyl/NHC complexes of
FeI and Fe0.14 In our continuing study on organoiron
complexes,15 we have synthesized half-sandwich FeII complexes
of NHCs, which activate the C−H bonds of heteroarenes and
benzene.15b,c Recently we have also reported FeII−dichloride
and FeII−dimethyl complexes of NHCs, which serve as good
catalyst precursors for the hydrosilylation of ketones.15e,f In this
study, we have examined reactive NHC complexes of low-
valent iron. Herein we report a new class of dinuclear Fe0
complexes having bulky NHCs and their reactions with CO and
1-azidoadamantane.
INTRODUCTION
■
Low-valent transition-metal complexes have been of interest
because of their utility in the activation of small molecules. For
example, low-coordinate complexes of low-valent iron (FeI or
lower) have been shown to reduce N2, leading to substantial
weakening or breaking of the NN bond.1 Some 16-electron
Fe0−phosphine complexes, such as Fe(PMe3)4 and Fe(dmpe)2
(dmpe = bis(dimethylphosphino)ethane), have been found to
promote intra- and intermolecular C−H bond activation.2 Low-
valent iron is probably also available in nature. [FeFe]-
hydrogenase, which catalyzes biological H2 production and
consumption, has a dinuclear iron−thiolate moiety in the active
site.3 This dinuclear unit is relevant to the known FeI−FeI
complexes (CO)3Fe(μ-SR)2Fe(CO)3,4 and the FeI−FeI and
FeII−FeI states have been suggested to be involved in the
catalytic cycle.5 One of the FeII−FeI models of [FeFe]-
hydrogenase carrying a redox-functional unit, [(CO)2(FcP*)-
Fe{μ-(SCH2)2NBn}Fe(dppv)(CO)]+ (FcP* = (C5Me5)Fe-
(C5Me4CH2PEt2), dppv = cis-bis(diphenylphosphino)ethene),
was found to undergo both proton reduction and H2 activation,
reproducing the enzymatic reactions.6 Notably, good π-
acceptor ligands such as CO and phosphines have been
commonly employed for the stabilization of low-valent iron
complexes.
RESULTS AND DISCUSSION
■
Synthesis and Structures of (NHC)FeCl3. The synthesis
of (NHC)FeCl3 was recently reported by Tonzetich et al.,16
and in situ generation of (NHC)FeX3 (X = F, Cl) for some
catalytic reactions has been reported by others.17 With a slight
modification of the procedure by Tonzetich et al., we also
prepared a series of NHC adducts of FeCl3. Treatment of
anhydrous FeCl3 with 1 equiv of NHCs in toluene afforded
(L)FeCl3 (1a, L = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene (IMes); 1b, L = 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene (IPr); 1c, L = 1,3-bis[(3,5-dimethylphenyl)-
methyl]imidazol-2-ylidene (IBnMe2); 1d, L = 1,3-diisopropyl-
4,5-dimethylimidazol-2-ylidene (IiPr)), in good to excellent
N-heterocyclic carbenes (NHCs) have received considerable
attention as ligands for transition metals.7 They are used as 2e-
donor ligands like phosphines; however, their stronger σ-
donating and weaker π-accepting abilities make NHCs unique.8
NHCs often make strong bonds with transition metals and
facilitate the synthesis of iron complexes, particularly in the FeII
state.9,10 On the other hand, NHC complexes of low-valent
iron have been relatively less explored. Previous examples
Received: October 25, 2013
© XXXX American Chemical Society
A
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isolated yields (72−97%) (Scheme 1). Complexes 1a−d,
including the known complex 1b,16 were all characterized by
Synthesis, Structures, and Properties of Dinuclear
Fe0−NHC Complexes. Reduction of (IMes)FeCl3 (1a) with
3.1 equiv of KC8 in THF afforded the dinuclear Fe0−NHC
complex Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a), which was
isolated as dark brown crystals in 39% yield. An analogous
complex containing IPr, Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b),
was similarly synthesized as dark brown crystals in 22% yield
(Scheme 2). The products from the reactions of 1c,d with KC8
remain uncharacterized.
Scheme 1
Scheme 2
1
elemental analysis, H NMR, and X-ray diffraction studies.
1
These complexes are paramagnetic, and their H NMR spectra
exhibited broad signals in the ranges of 50.8−2.4 ppm (1a),
53.0−2.1 ppm (1b), 53.0−2.5 ppm (1c), and 85.2−4.5 ppm
(1d).
The molecular structures of 1a−d were determined by X-ray
diffraction and are shown in Figure 1 for 1a,c and Figures S1
Figure 2. Molecular structure of Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a)
with thermal ellipsoids at the 50% probability level. Selected bond
distances (Å) and angle (deg): Fe−Fe*, 2.6214(6); Fe−C1,
1.9792(18); Fe−C(η6-arene), 2.0202(19)−2.235(3); C1−Fe−Fe*,
89.08(7).
Figure 1. Molecular structures of (IMes)FeCl3 (1a) (top) and
(IBnMe2)FeCl3 (1c) (bottom) with thermal ellipsoids at the 50%
probability level. Selected bond distances (Å) and angles (deg) for 1a:
Fe−C1, 2.091(4); Fe−Cl, 2.1770(13)−2.1827(12), 2.1800 (av); C1−
Fe−Cl, 106.48(10)−110.53(10), 108.99 (av); Cl−Fe−Cl, 109.44(5)−
110.37(5), 109.95 (av). Selected bond distances (Å) and angles (deg)
for 1c: Fe−C1, 2.081(3); Fe−Cl, 2.1710(11)−2.1881(11), 2.1772
(av); C1−Fe−Cl, 105.62(8)−114.96(10), 109.54 (av); Cl−Fe−Cl,
108.57(5)−109.82(5), 109.37 (av).
and S2 for 1b,d (Supporting Information). The iron atoms in
complexes 1a−d are in a nearly ideal tetrahedral geometry with
one NHC ligand and three chlorides, and the C−Fe−Cl and
Cl−Fe−Cl angles range from 103.72(7) to 114.96(10)°. The
Fe−C(carbene) distances are 2.081(3)−2.093(3) Å, which are
comparable to those reported for tetrahedral FeIII−NHC
complexes (1.909(4)−2.174(2) Å).16,18
Figure 3. Molecular structure of Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b)
with thermal ellipsoids at the 50% probability level. Selected bond
distances (Å) and angle (deg): Fe−Fe*, 2.5832(5); Fe−C1, 1.975(2);
Fe−C(η6-arene), 2.0256(19)−2.203(3); C1−Fe−Fe*, 88.77(6).
B
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As shown in Figures 2 and 3, X-ray diffraction studies for
2a,b reveal their unique dinuclear structures, each consisting of
two Fe0 centers and NHC ligands. The iron center is
coordinated by the carbene carbon and one of the arene
rings of another NHC ligand in an η6 fashion. An analogous
dinuclear structure with a μ-η1(C):η6(arene) coordination
mode of NHC was found in the Ni0 complex Ni2{μ-
η1(C):η6(iPr2C6H3)-IPr}2.19 Each nickel atom of this dinuclear
complex possesses 18 electrons, and there is no Ni−Ni bonding
interaction (Ni···Ni, 3.568(2) Å). In contrast, the iron atoms in
2a,b have 16 electrons, and the Fe−Fe distances are relatively
short: 2.6214(6) Å (2a) and 2.5832(5) Å (2b). The Fe−C(η6-
arene) distances of 2a (2.0202(19)−2.235(3) Å) and 2b
(2.0250(19)−2.208(3) Å) are comparable to reported Fe0−
C(η6-arene) distances (1.990−2.238 Å).20
Figure 5. Possible d-orbital interactions in 2a,b.
Interestingly, the magnetic properties of 2a,b are distinctively
different. Complex 2b is diamagnetic, and the 1H NMR
spectrum exhibited sharp signals in the range 7.79−0.66 ppm.21
1
In contrast, complex 2a is paramagnetic, and its H NMR in
C6D6 exhibited broad signals in the range from +17.8 to −9.47
ppm.21 The solution magnetic moment of 2a in C6D6 obtained
from the Evans method22 was μeff = 4.4 μB at 297.7 K. The
temperature dependence of magnetization of 2a was also
measured at 2−300 K (Figure 4). The effective magnetic
Figure 6. Structures of 2a (left) and 2b (right), highlighting the planes
defined by iron atoms, carbene carbons, and the centroids of iron-
bound aromatic rings.
is 36.34(9)°. The C−Fe−Fe*−C* torsion angle of 2a is
156.34(6)°. As can be seen from Figure 6 (left, top and
bottom), the Fe−Fe* vector of 2a is out of both xy planes, due
to the distortion between two Fe(IMes) units. Such distortion
2
2
in 2a possibly results in less efficient interactions of dx −y and
dyz orbitals between two iron centers (Figure 5 right), leading
to the S = 2 state. On the other hand, the orbital interactions
are efficient in 2b, allowing the S = 0 state. The distortion
between two Fe(IMes) units of 2a could be due to steric
congestion between the mesityl groups, as the 4-methyl group
of the η6-mesityl moiety is near the noncoordinating mesityl
moiety of another IMes ligand.
The cyclic voltammograms for 2a,b were measured in THF
at room temperature using [nBu4N][PF6] as an electrolyte. As
shown in Figure 7, a quasi-reverse redox couple was observed at
E1/2 = −1.50 V (2a) and −1.38 V (2b) vs Ag/Ag+. Bulk
electrolysis suggested that these redox couples are 2e processes,
Figure 4. Temperature dependence of magnetization of 2a in the solid
state. Inset: the inverse molar magnetic susceptibility of 2a.
moment of 2a is nearly constant (μeff = 5.14−5.27 μB) in the
temperature range 50−300 K. As shown in the inset of Figure
4, a good fit to Curie−Weiss behavior was obtained for 50−300
K, exhibiting a linear dependence of the inverse of the magnetic
susceptibility as a function of temperature. The observation of
Curie−Weiss behavior at 50−300 K indicates the paramagnetic
nature in this temperature range. The slope of the Curie−Weiss
plot gives a Curie constant of 3.31 mol emu−1 K−1. The
effective magnetic moment of 2a calculated from the Curie
constant is μeff = 5.15 μB, which is close to the spin-only value
for an S = 2 state (4.90 μB).
The significant difference in magnetic properties of 2a,b can
be attributed to the extent of d-orbital interactions between two
iron centers. The d orbitals which can be used for the Fe−Fe
2
2
interaction in 2a,b are dx −y and dyz, as shown in Figure 5 (left),
where the x axis lies along the Fe−C(carbene) bond and the xy
plane is defined by Fe, C(carbene), and the centroid of the
iron-bound η6-arene ring. The xy planes are highlighted in red
and blue in Figure 6 (top). The two xy planes in 2b are almost
coplanar with an interplane angle of 0.40(8)°, while that of 2a
Figure 7. Cyclic voltammograms of 2a (left) and 2b (right) in THF
with 0.2 M [nBu4N][PF6] as supporting electrolyte.
C
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probably between the Fe0Fe0 and FeIFeI states. The appearance
of the 2e process may be due to the instability of the mixed-
valence Fe0FeI state. The easier oxidation of 2a in comparison
to that of 2b is in agreement with the aforementioned less
efficient d-orbital interactions in 2a leading to the higher-lying
d electrons. Attempts to synthesize the oxidized forms of 2a,b
by treatment with 1 or 2 equiv of [Cp2Fe](PF6) have been
unsuccessful, giving uncharacterizable brown solids.
Reactions of Dinuclear Fe0−NHC Complexes. (a). CO.
While the iron centers in complexes 2a,b are efficiently
stabilized by the μ-η1(C):η6(arene)-NHC ligands, these
complexes can be seen as a dimeric form of (NHC)Fe0.
Indeed, 2a,b were found to split into monomeric species in the
presence of CO. When a toluene solution of 2a or 2b was
exposed to 1 atm of CO at room temperature, the color of the
solutions changed from dark brown to yellow, forming the
monomeric 18-electron complexes (L)Fe(CO)4 (3a, L = IMes;
3b, L = IPr) in 83% (3a) and 85% (3b) yields, respectively
(Scheme 3). The same iron complexes were recently prepared
Figure 8. Molecular structure of (IMes)Fe(CO)4 (3a) with thermal
ellipsoids at the 50% probability level. Selected bond distances (Å) and
angles (deg): Fe−C1, 1.9945(18); Fe−C2, 1.780(2); Fe−C3,
1.798(3); Fe−C4, 1.794(3); Fe−C5, 1.792(3); C1−Fe−C2,
174.72(9); C1−Fe−C3, 95.21(9); C1−Fe−C4, 84.50(9); C1−Fe−
C5, 95.89(8); C3−Fe−C4, 121.11(11); C3−Fe−C5, 119.52(10);
C4−Fe−C5, 119.06(10).
infrared spectra in hexane, 3a,b exhibit four CO bands at 2041,
1960, 1935, 1921 cm‑1 (3a) and 2042, 1962, 1936, 1923 cm‑1
(3b), respectively.
(b). 1-Azidoadamantane. Some low-valent iron complexes
have been shown to activate organoazides (N3R) to provide
iron−imido complexes.13b,18,23,24 For example, Peters et al.
have reported the reaction of the FeI complex [PhBP3]Fe-
(PPh3) ([PhBP3] = [PhB(CH2PPh2)3]−) with p-tolyl azide,
where the FeIII−imido complex [PhBP3]Fe(N-p-tolyl) was
obtained.23a Smith et al. have synthesized a relevant FeIII−
imido complex stabilized by a tris(carbene)−borate ligand,
LMesFe(NAd) (LMes = phenyltris(1-mesitylimidazol-2-ylidene)-
borate)),18b from the reaction of the FeI complex LMesFe(η2-
C8H14) with 1-azidoadamantane (AdN3).
Scheme 3
Treatment of a THF solution of 2a with 4 equiv of 1-
azidoadamantane resulted in the formation of a dinuclear iron−
imido complex having a tetrazene (RNNNNR) ligand, (η2-
N4Ad2)Fe(μ-NAd)2Fe(IMes) (4), in 30% yield (Scheme 4).
Complex 4 is paramagnetic, and its 1H NMR spectrum
exhibited broad signals in the range from +22.5 to −12.7 ppm.
The magnetic susceptibility of 4 in the solid state (Figure S5,
Supporting Information) reveals that the effective magnetic
moment gradually decreases as the temperature is lowered,
from 4.76 μB at 300 K to 3.61 μB at 50 K.
Scheme 4
from the reactions of Fe3(CO)12 with imidazolium salts or of
Fe(CO)5 with L,14f,g and analogous iron complexes with the
different NHC ligands IMe (1,3-dimethylimidazol-2-ylidene)
and SIMe (1,3-dimethylimidazolin-2-ylidene) have also been
14a
reported.14 (IMe)Fe(CO)4 was synthesized via deprotona-
tion of the imidazolium salt IMe·HI with the hydride complex
14c
K{FeH(CO)4}, while (SIMe)Fe(CO)4 was prepared from
the reaction of Fe(CO)5 with (SIMe)2.
The molecular structures of 3a,b were determined by X-ray
crystallography and are shown in Figure 8 and Figure S3
(Supporting Information). The coordination geometry of iron
in 3a,b is slightly distorted trigonal bipyramidal, with an NHC
ligand occupying the apical position. The C(carbene)−Fe−
Cap(carbonyl) angles (3a, 174.72(9)°; 3b, 170.67(9)°) are
slightly narrowed from the ideal trigonal-bipyramidal geometry.
The Fe−C(carbene) distances of 1.9945(18) Å (3a) and
2.0049(17) Å (3b) are similar to that of (IMe)Fe(CO)4
(2.007(5) Å).14b In the 13C{1H} NMR spectra of 3a,b, a single
carbonyl resonance was observed at δ 217.2 (3a) and δ 215.5
(3b), suggesting that Berry pseudorotation exchanges apical
and equatorial positions within the NMR time scale. In the
D
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The molecular structure of 4 was determined by X-ray
analysis (Figure 9). The iron centers, imido nitrogen atoms,
FeII centers, (IiPr)Fe(μ2-NDipp)2Fe(IiPr)18d (Dipp = 2,6-
diisopropylphenyl; Fe−C(carbene) = 2.068(3) Å and Fe−
N(imido) = 1.867(2) Å). In contrast, the Fe2−N(tetrazene)
distances (1.8287(17) and 1.8321(15) Å) are clearly shorter
than those reported for tetrahedral FeII complexes having a
tetrazene ligand (2.0089(7) and 1.9556(8) Å).26 The Fe2−
N(imido) distances (1.7936(15) and 1.7849(17) Å) are also
shorter than the Fe1−N(imido) distances (1.8175(17) and
1.8200(15) Å). Thus, the oxidation states of the iron centers
would be Fe1II−Fe2IV.
Scheme 5 illustrates a possible reaction pathway for the
formation of 4. The reaction of 2a with 2 equiv of AdN3 would
Scheme 5
Figure 9. Molecular structure of (IMes)Fe(μ-NAd)2Fe(η2-N4Ad2) (4)
with thermal ellipsoids at the 50% probability level. Selected bond
distances (Å) and angles (deg): Fe1−Fe2, 2.3906(5); Fe1−C1,
2.0252(19); Fe1−N1, 1.8175(17); Fe1−N2, 1.8200(15); Fe2−N1,
1.7936(15); Fe2−N2, 1.7849(17); Fe2−N3, 1.8287(17); Fe2−N6,
1.8321(15); N3−N4, 1.351(2); N4−N5, 1.289(3); N5−N6, 1.354(3);
C1−Fe1−N1, 131.49(8); C1−Fe1−N2, 132.44(8); N1−Fe1−N2,
95.95(7); N1−Fe2−N2, 98.07(7); N1−Fe2−N3, 121.29(8); N3−
Fe2−N6, 79.00(7); N2−Fe2−N6, 120.24(8).
and the carbene carbon are coplanar, with the largest deviation
of 0.082(2) Å at the carbene carbon. The Fe−Fe distance of
2.3906(5) Å is shorter than the Fe−Fe separation found in
metallic iron (2.48 Å)25 and suggests the presence of an Fe−Fe
bonding interaction. While Fe1 is trigonal planar, Fe2 is
tetrahedral with two bridging imido ligands and a bidentate 1,4-
bis(adamantyl)tetrazene (Ad2N4) ligand. The five-membered
metallacycle consisting of Ad2N4 and Fe is planar, with the
largest deviation of 0.015(2) Å for N6. The two Fe2−N bond
distances in the metallacycle are 1.8287(17) Å (Fe2−N3) and
1.8321(15) Å (Fe2−N6). The two N−N bonds neighboring
the Fe2−N bonds are 1.351(2) Å (N3−N4) and 1.354(3) Å
(N5−N6), and the N4−N5 distance of 1.289(3) Å is shorter
than the N3−N4 and N5−N6 distances. The trend of these
N−N distances found in the Ad2N4 tetrazene unit is indicative
of the dianionic form shown in Figure 10A, with two N−N
lead to the formation of the dinuclear FeII−imido intermediate
A, which is an analogue of (IiPr)Fe(μ-NDipp)2Fe(IiPr).18d The
reaction of A with AdN3 would follow, if dissociation of one of
the IMes ligands from A occurs, due to the possible steric
repulsion between the μ-NAd ligands and the mesityl groups of
IMes ligands.27 The resultant intermediate B has a terminal
imido ligand on a low-coordinate iron center, and the Fe
NAd moiety of the tentative species B may further take up
AdN3 to undergo [3 + 2] cyclization in a manner analogous to
the “click chemistry” between alkynes and organoazides, to
provide the tetrazene moiety of 4.28
Figure 10. Electronic configurations for a bidentate tetrazene ligand.
Concluding Remarks. The first homoleptic NHC
complexes of Fe0, Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a) and
Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b), were synthesized from
the reduction of FeIII−chloride precursors. The Fe0 centers in
2a,b are stabilized by a unique μ-η1(C):η6(arene) coordination
mode of the NHC. A quasi-reversible 2e redox event between
the Fe0Fe0 and FeIFeI states was observed for both 2a and 2b.
The dimeric complexes 2a,b are reactive, and they were found
to serve as precursors for a monomeric (NHC)Fe0 moiety in
reactions with CO. Complex 2a was also found to activate 1-
azidoadamantane, giving rise to the dinuclear iron−imido
complex 4 having a tetrazene ligand.
single bonds (N3−N4 and N5−N6) and an NN double
bond (N4N5). Other possible configurations for the five-
membered metallacycle shown in Figure 10 include the
monoanionic radical form (B) and the neutral form (C),26
whereas these forms do not fit with the trend of N−N distances
in the Ad2N4 ligand of 4. The presence of a dianionic Ad2N4
ligand and two μ-NAd ligands in the dinuclear iron complex 4
leads to the FeIIIFeIII or FeIIFeIV oxidation state. The bond
distances around Fe1, Fe1−C(carbene) (2.0252(19) Å) and
Fe1−N(imido) (1.8175(17) and 1.8200(15) Å), are close to
those of a dinuclear μ-imido complex having trigonal-planar
E
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mL × 3) to afford a yellow powder of 1b (1.99 g, 87%). Single crystals
EXPERIMENTAL SECTION
■
suitable for crystallography were obtained from an Et2O solution at
General Procedures. All reactions were carried out using standard
Schlenk techniques and a glovebox under a nitrogen or an argon
atmosphere. Toluene, diethyl ether, THF, and hexane were purified by
the method of Grubbs,29 where the solvents were passed over columns
of activated alumina and a supported copper catalyst supplied by
Hansen & Co. Ltd. Other solvents were degassed and distilled from
sodium benzophenone ketyl. Deuterated solvents, C61D6 and d8-THF,
were dried by sodium and distilled prior to use. The H and 13C{1H}
NMR spectra were recorded on a JEOL ECA-600 instrument. The
signals were referenced to the residual peak of the deuterated solvent.
Solution magnetic susceptibility was determined by the Evans method,
using C6D6/hexamethylbenzene or d8-THF/tetramethylsilane solu-
tions.22 The solid-state magnetic susceptibility was measured using a
Quantum Design MPMS-XL SQUID-type magnetometer, and the
samples were sealed in quartz tubes. UV−vis spectra were measured
on a JASCO V560 spectrometer. Cyclic voltammetry measurements
were performed in a single-compartment cell under a nitrogen
atmosphere at 23 °C using a BSA-660B electrochemical analyzer. A
three-electrode setup was employed, comprising of a glassy-carbon
working electrode, platinum-wire auxiliary electrode, and Ag/AgCl
quasi-reference electrode with 0.2 M [Bu4N][PF6] as the supporting
electrolyte. The bulk electrolysis of 2a,b was performed at a potential
of −1.2 V (2a) or −1.0 V (2b) vs Ag/Ag+ with THF (3−5 mL)
solutions of Fe(0) complexes (10−20 mg) in the presence of 0.2 M
[Bu4N][PF6] as the supporting electrolyte. Elemental analyses were
performed on a LECO CHNS-932 microanalyzer, where the samples
were sealed into silver or tin capsules in a glovebox. Purification of
complex 5 was performed on an LC908 preparative HPLC (Japan
Analytical Industry) eluted with toluene under a nitrogen atmosphere.
The two HPLC columns (20 mm × 600 cm each) were filled with Bio-
Beads S-X1 (Bio-Rad laboratories). X-ray diffraction data were
collected on a Rigaku AFC8 or a Rigaku RA-Micro7 instrument
equipped with a CCD area detector by using graphite-monochromated
Mo Kα radiation. N-heterocyclic carbenes (IMes, IPr, and IiPr)30 and
1
−40 °C. H NMR (600 MHz, C6D6): δ 53.0 (2H), 8.01 (6H), 5.50
(4H), 2.11 (24H). Anal. Calcd for C27H36N2FeCl3: C, 58.88; H, 6.59;
N, 5.09. Found: C, 58.88; H, 6.57; N, 5.03.
Synthesis of (IBnMe2)FeCl3 (1c). A toluene (10 mL) solution of
IBnMe2 (180 mg, 0.60 mmol) was added to a toluene (30 mL)
suspension of FeCl3 (97 mg, 0.60 mmol), and the mixture was stirred
overnight at room temperature. After centrifugation, the orange
solution was evaporated until dryness, and the residue was washed
with hexane (10 mL × 3) to afford a yellow powder of 1c (200 mg,
72%). Single crystals suitable for crystallography were obtained from a
toluene solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 53.0 (2H),
9.55 (2H), 6.76 (4H), 3.38 (4H), 2.53 (12H). Anal. Calcd for
C21H24N2FeCl3: C, 54.05; H, 5.18; N, 6.00. Found: C, 53.74; H, 5.19;
N, 6.06.
Synthesis of (IiPr)FeCl3 (1d). A toluene (30 mL) solution of IiPr
(1.62 g, 8.97 mmol) was added to a toluene (30 mL) suspension of
FeCl3 (1.46 g, 8.97 mmol), and the mixture was stirred overnight at
room temperature. After centrifugation, the purple solution was
evaporated under reduced pressure, and the residue was washed with
hexane (30 mL × 3) to afford a dark purple powder of 1d (3.00 g,
97%). Single crystals suitable for crystallography were obtained from a
toluene solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 85.2 (6H),
15.4 (12H), 4.45 (2H). Anal. Calcd for C11H20N2FeCl3: C, 38.58; H,
5.89; N, 8.18. Found: C, 38.12; H, 6.16; N, 8.20.
Synthesis of Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a).
Complex 1a (300 mg, 0.64 mmol) and KC8 (280 mg, 2.07 mmol)
were charged into a flask, and the mixture was cooled to −197 °C.
THF (30 mL) was condensed onto the mixture using a vacuum-
transfer technique. The mixture was gradually warmed to room
temperature and stirred overnight. After centrifugation, the dark red
solution was evaporated until dryness, and the residue was washed
with hexane (10 mL × 3) and extracted with Et2O (10 mL). After
filtration, the solution was cooled at −40 °C to afford dark brown
crystals of 2a (90 mg, 39%). Single crystals suitable for crystallography
were obtained by diffusion of hexane into the THF extract at −40 °C.
1H NMR (600 MHz, C6D6): δ 17.8 (4H, b), 7.68 (4H, g), 7.66 (2H, d
or e), 2.83 (12H, f), 2.77 (2H, d or e) 2.62 (6H, h), −1.96 (12H, c),
31
KC8 were prepared according to literature procedures.
Preparation of IBnMe2. KOtBu (1.409 g, 12.6 mmol) was added to
a THF (50 mL) solution of imidazole (0.855 g, 12.6 mmol) at room
temperature. The resulting white suspension was then stirred for 3 h
before addition of a THF (50 mL) solution of 3,5-dimethylbenzyl
bromide (5.00 g, 25.1 mmol) at 0 °C. The mixture was stirred for 3 h
at room temperature and then filtered to remove KBr. The solvent was
removed under reduced pressure and the residue washed with diethyl
ether (2 × 20 mL). The crude imidazolium salt IBnMe2·HBr was
obtained as a white solid (4.53 g, 93%). KOtBu (0.22 g, 1.96 mmol)
was added to a THF (30 mL) suspension of IBnMe2·HBr (0.70 g, 1.82
mmol), and the resulting mixture was stirred overnight. After
centrifugation, the solvent was removed under reduced pressure, and
the residue was extracted with toluene (15 mL). The orange solution
was slowly evaporated to dryness. The resultant solid was washed with
hexane (10 mL) to afford IBnMe2 (0.35 g, 63%) as a light yellow
−9.47 (6H, a). Solution magnetic susceptibility (C6D6, 297.7 K): μeff
=
4.4 μB. Cyclic voltammogram (2 mM in THF): E1/2 = −1.50 V (vs Ag/
Ag+). UV (THF, room temperature): λmax 379 nm (ε 15000), 471 nm
(sh, ε 6600). Anal. Calcd for C42H48N2Fe2: C, 70.01; H, 6.71; N, 7.77.
Found: C, 69.93; H, 6.77; N, 7.79.
1
Synthesis of Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b).
powder. H NMR (600 MHz, C6D6): δ 6.86 (s, 2H), 6.70 (s, 1H),
6.48 (s, 1H), 5.14 (s, 4H), 2.05 (s, 12H).
Synthesis of (IMes)FeCl3 (1a). A toluene (40 mL) solution of
IMes (0.938 g, 3.08 mmol) was added to a toluene (100 mL)
suspension of FeCl3 (0.500 g, 3.08 mmol), and the mixture was stirred
overnight at room temperature. After centrifugation, the solvent was
removed under reduced pressure, and the resultant solid was washed
with hexane (20 mL × 3) and then Et2O (20 mL × 3) to afford an
orange powder of 1a (1.39 g, 97%). Single crystals suitable for
crystallography were obtained by diffusion of hexane into the THF
Complex 1b (300 mg, 0.54 mmol) and KC8 (225 mg, 1.66 mmol)
were charged into a flask, and the mixture was cooled to −197 °C.
THF (40 mL) was condensed onto the mixture using a vacuum-
transfer technique. The mixture was gradually warmed to room
temperature and stirred overnight. After centrifugation, the dark red
solution was evaporated until dryness, and the residue was washed
with hexane (10 mL × 3) and Et2O (10 mL) and then extracted with
THF (3 mL). After filtration, diffusion of hexane into the filtrate at
−40 °C afforded dark brown crystals of 2b (54 mg, 22%). Single
1
extract. H NMR (600 MHz, C6D6): δ 50.8 (2H), 4.28 (12H), 2.80
(6H), 2.36 (4H). Anal. Calcd for C21H24N2FeCl3: C, 54.05; H, 5.18;
N, 6.00. Found: C, 54.08; H, 5.15; N, 5.96.
Synthesis of (IPr)FeCl3 (1b).16 A toluene (50 mL) solution of IPr
(1.555 g, 4.00 mmol) was added to a toluene (100 mL) suspension of
FeCl3 (0.650 g, 4.01 mmol), and the mixture was stirred overnight at
room temperature. After centrifugation, the orange solution was
evaporated until dryness, and the residue was washed with hexane (30
F
dx.doi.org/10.1021/om401039z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
crystals suitable for crystallography were obtained from an Et2O
solution at −40 °C. H NMR (600 MHz, C6D6): δ 7.79 (d, 4H, k),
Table 1. Crystal Data and Summary of Refinement for
Complexes 1a−d, 2a,b, 3a,b, and 4
1
7.73 (t, 2H, l), 7.34 (s, 2H, f or g), 7.08 (s, 2H, f or g), 5.57 (s, 2H, a),
4.77 (s, 4H, b or d), 4.69 (s, 4H, b or d), 1.73 (d, 12H, h or j), 1.60 (d,
16H, h or j, (i), 0.89 (s, 12H, c or e), 0.66 (s, 12H, c or e). 13C{1H}
NMR (150 MHz, d8-THF): δ 149.2, 142.2, 129.5, 128.3, 124.3, 120.4,
104.5, 86.0, 81.9, 75.4, 31.3, 29.6, 25.4, 23.0, carbene carbon signal could
not be detected. Cyclic voltammogram (2 mM in THF): E1/2 = −1.38 V
(vs Ag/Ag+). UV (THF, room temperature): λmax 387 nm (ε 17000),
491 nm (sh, ε 4900). Anal. Calcd for C54H72N2Fe2: C, 72.97; H, 8.16;
N, 6.30. Found: C, 72.65; H, 8.11; N, 6.25.
1a
1b
1c
formula
fw
C21H24Cl3FeN2
466.64
C27H36Cl3FeN2
550.80
C21H24Cl3FeN2
466.64
temp (°C)
cryst syst
space group
a (Å)
−100
−100
−100
triclinic
orthorhombic
Pca21 (No. 29)
16.358(2)
16.636(2)
17.004(2)
monoclinic
P21/c (No. 14)
18.235(2)
16.233(2)
20.160(3)
P1 (No. 2)
̅
8.474(2)
9.880(3)
14.618(3)
74.080(13)
73.54(2)
75.082(12)
1107.0(5)
2
b (Å)
Synthesis of (IMes)Fe(CO)4 (3a). A toluene (15 mL) solution of
2a (30 mg, 0.042 mmol) was exposed to 1 atm of CO at −78 °C. The
mixture was gradually warmed to room temperature and stirred
overnight. The solvent was removed under reduced pressure, and the
residue was extracted with hexane (10 mL). After filtration, the yellow
solution was evaporated until dryness, which gave 3a (31 mg, 83%) as
a light yellow powder. Single crystals suitable for crystallography were
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
102.897(2)
4627.4(9)
8
5816.8(11)
8
1
obtained from a toluene solution at −40 °C. H NMR (600 MHz,
Dcalcd (g/cm3)
1.340
0.0367
0.0812
1.003
1d
1.258
0.0478
0.0837
1.005
2a
1.400
C6D6): δ 6.77 (s, 4H, CH of m-Mes), 6.13 (s, 2H, CH of CN2C2H2),
2.10 (s, 6H, CH3 of p-Mes), 2.03 (s, 12H, CH3 of o-Mes). 13C NMR
(150 MHz, C6D6): δ 217.2, 189.9, 139.5, 137.2, 136.0, 129.7, 124.3,
21.6, 18.1. IR (hexane, cm−1): ν(CO) 2041 (m), 1960 (m), 1935 (s),
1921 (s). Anal. Calcd for C25H24N2O4Fe: C, 63.57; H, 5.12; N, 5.93.
Found: C, 63.16; H, 5.40; N, 5.62.
a
R1
0.0407
b
wR2
0.0838
c
GOF
1.002
2b
formula
fw
C11H20Cl3FeN2
342.50
C42H48Fe2N4
720.56
C54H72Fe2N4
888.88
Synthesis of (IPr)Fe(CO)4 (3b). A toluene (15 mL) solution of 2b
(30 mg, 0.034 mmol) was exposed to 1 atm of CO at −78 °C. The
mixture was gradually warmed to room temperature and stirred
overnight. The solvent was removed under reduced pressure, and the
resultant solid was extracted with hexane (10 mL). After filtration, the
yellow solution was evaporated until dryness, which gave 3b (32 mg,
85%) as a light yellow powder. Single crystals suitable for
temp (°C)
cryst syst
space group
a (Å)
−100
−100
−150
orthorhombic
Pbca (No. 61)
16.021(2)
11.689(2)
17.185(3)
monoclinic
C2/c (No. 15)
21.810(5)
8.318(2)
21.827(4)
109.513(3)
3732.4(13)
4
monoclinic
P21/n (No. 14)
16.732(2)
14.9184(11)
21.021(2)
110.9691(11)
4899.7(7)
4
b (Å)
1
c (Å)
crystallography were obtained from a hexane solution at −40 °C. H
NMR (600 MHz, C6D6): δ 7.27 (t, 2H, CH of p-Ar), 7.12 (d, 4H, CH
of m-Ar), 6.57 (s, 2H, CH of CN2C2H2), 2.71 (sept, 4H, CH of o-Ar),
1.41 (d, 12H, CHCH3 of o-Ar), 0.99 (d, 12H, CHCH3 of o-Ar). 13C
NMR (150 MHz, C6D6): δ 215.5, 191.3, 146.3, 136.7, 130.6, 125.4,
124.3, 28.6, 25.6, 22.4. IR (hexane, cm−1): ν(CO) 2042 (m), 1962
(m), 1936 (s), 1923 (s). Anal. Calcd for C31H36N2O4Fe: C, 66.91; H,
6.52; N, 5.03. Found: C, 66.90; H, 6.58; N, 5.07.
β (deg)
V (Å3)
Z
3218.4(7)
8
Dcalcd (g/cm3)
1.414
0.0409
0.0724
1.004
3a
1.282
1.205
a
R1
0.0371
0.0376
b
wR2
0.0696
0.0788
c
Synthesis of (IMes)Fe(μ-NAd)2Fe(η2-N4Ad2) (4). A THF (10
mL) solution of N3Ad (150 mg, 0.85 mmol) was added to a THF (20
mL) solution of 2a (150 mg, 0.21 mmol) and the mixture stirred
overnight at room temperature. The solvent was removed under
reduced pressure, and the resultant brown solid was washed with
hexane (3 mL × 3) and extracted with toluene (8 mL). After filtration,
the brown filtrate was evaporated until dryness. The residue was again
dissolved in toluene (2.5 mL), and the solution was injected into a
preparative size-exclusive HPLC under N2 and was eluted with toluene
at a flow rate of 4.0 mL/min. The elution time for 4 was 58−66 min.
The solution was evaporated to dryness to give 4 (65 mg, 30%) as a
brownish green solid. Single crystals suitable for crystallography were
GOF
1.004
1.004
3b
4·2C4H8O
formula
fw
C25H24FeN2O4
472.32
C31H36FeN2O4
556.48
C69H100Fe2N8O2
1185.30
temp (°C)
cryst syst
space group
a (Å)
−100
−100
−160
monoclinic
P21/c (No. 14)
18.420(3)
16.442(3)
7.640(2)
99.650(3)
2281.2(7)
4
monoclinic
P21/c (No. 14)
17.345(5)
9.287(3)
18.211(5)
98.590(5)
2901(2)
4
monoclinic
P21/n (No. 14)
14.106(2)
23.925(4)
19.691(3)
110.488(2)
6225(2)
4
b (Å)
c (Å)
β (deg)
V (Å3)
Z
1
obtained from a THF solution at −40 °C. H NMR (600 MHz, d8-
THF): δ 22.5, 15.0, 10.2, 6.1, 2.8, −1.5, −1.9, −7.0, −12.7. UV (THF,
room temperature): three shoulders were observed at 430, 560, and
730 nm. Anal. Calcd for C61H84N8Fe2: C, 70.38; H, 8.13; N, 10.76.
Found: C, 70.36; H, 8.52; N, 10.67.
Dcalcd (g/cm3)
1.375
1.274
1.265
a
R1
0.0386
0.0408
0.0448
b
wR2
0.0813
0.0819
0.0901
X-ray Crystal Structure Determination. Crystal data and
refinement parameters for 1a−d, 2a,b, 3a,b, and 4 are summarized
in Table 1. Single crystals were coated with oil (Immersion Oil, type B:
Code 1248, Cargille Laboratories, Inc.) and mounted on loops.
Diffraction data were collected at −100, −150, or −160 °C under a
cold nitrogen stream on a Rigaku RA-Micro7 equipped with a
Saturn70 CCD detector, using graphite-monochromated Mo Kα
radiation (λ = 0.710690 Å). Six preliminary data frames were measured
at 0.5° increments of ω, to assess the crystal quality and preliminary
unit cell parameters. The intensity images were also measured at 0.5°
intervals of ω. The frame data were integrated using the CrystalClear
program package, and the data sets were corrected for absorption
using a REQAB program. The calculations were performed with the
c
GOF
1.005
1.008
1.005
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo| (I > 2σ(I)). wR2 = [(∑(w(|Fo| − |Fc|)2/
c
2
∑wFo ))1/2 (all reflections). GOF = [∑w(|Fo| − |Fc|)2/(No − Nv)]1/2
(where No = number of observations, Nv = number of variables).
CrystalStructure program package. All structures were solved by direct
methods and refined by full-matrix least-squares. Anisotropic refine-
ment was applied to all non-hydrogen atoms except for disordered
atoms (refined isotropically), and all hydrogen atoms were placed at
calculated positions. In the asymmetric units of 1a,b, there are two
crystallographically independent molecules. Additional data are
available as Supporting Information.
G
dx.doi.org/10.1021/om401039z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Gallagher, M. K.; Cowie, M.; Hames, B. W.; Fackler, J. P.; Mazany, A.
M. Organometallics 1987, 6, 283. (d) Kramer, A.; Lingnau, R.; Lorenz,
I.-P.; Mayer, H. A. Chem. Ber. 1990, 123, 1821. (e) Cody, G. D.;
Boctor, N. Z.; Filley, T. R.; Hazen, R. M.; Scott, J. H.; Sharma, A., Jr.
Science 2000, 289, 1337. (f) Volkers, P. I.; Boyke, C. A.; Chen, J.;
Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R.; Yao, H. Inorg. Chem.
2008, 47, 7002.
(5) (a) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y.
Chem. Rev. 2007, 107, 4273. (b) Tard, C.; Pickett, C. J. Chem. Rev.
2009, 109, 2245.
(6) Camara, J. M.; Rauchfuss, T. B. Nat. Chem. 2012, 4, 26.
(7) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem.
Rev. 2000, 100, 39. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002,
41, 1290. (b) Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics
2003, 22, 612. (c) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J.
Organomet. Chem. 2005, 690, 5407. (d) Nolan, S. P. N-Heterocyclic
Carbenes in Synthesis; Wiley-VCH: Weinheim, Germany, 2006.
(e) Glorius, F. N-Heterocyclic Carbenes in Transition-Metal Catalysis;
Springer: New York, 2007; Topics in Organometallic Chemistry 21.
(f) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251, 841.
ASSOCIATED CONTENT
* Supporting Information
Figures giving magnetic susceptibility measurements for
complexes 2b and 4 and ORTEP diagrams for complexes
1b,d and 3b and CIF files giving crystallgraphic data for
complexes 1a−d, 2a,b, 3a,b, and 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: ohki@chem.nagoya-u.ac.jp (Y.O.); i45100a@nucc.cc.
nagoya-u.ac.jp (K.T.).
■
Present Addresses
†National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba Central 5-2, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan.
‡Department of Chemistry, Graduate School of Science, Osaka
University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-
0043, Japan.
́
(g) Dıez-Gonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874.
(h) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int.
Ed. 2007, 46, 2768. (i) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int.
Notes
The authors declare no competing financial interest.
Ed. 2008, 47, 3122. (j) Wurtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41,
̈
1523. (k) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo,
ACKNOWLEDGMENTS
■
́
́
L. Coord. Chem. Rev. 2009, 253, 687. (l) Dıez-Gonzalez, S.; Marion,
This research was financially supported by Grants-in-Aid for
Scientific Research (Nos. 23000007, 23685015, 25105725,
25109522) from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. We are grateful to Prof.
Kunio Awaga, Prof. Michio Matsushita, Prof. Hirofumi
Yoshikawa, and Dr. Yoshiaki Syuku (Nagoya Univeristy) for
aiding us with a SQUID-type magnetometer. We also thank
Prof. Kohei Tamao, Prof. Tsukasa Matsuo, and Dr. Mikinao Ito
(RIKEN and Kinki University) for their advise on HPLC
purification.
N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (n) Gusev, D. G.
Organometallics 2009, 28, 6458. (m) Clavier, H.; Nolan, S. P. Chem.
Commun. 2010, 841. (o) Droge, T.; Glorius, F. Angew. Chem., Int. Ed.
̈
2010, 122, 7094.
(8) The π-accepting properties of NHC ligands have been suggested
for group 11 metal complexes: (a) Hu, X.; Tang, Y.-J.; Gantzel, P.;
Meyer, K. Organometallics 2003, 22, 612−614. (b) Hu, X.; Castro-
Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755−
764.
(9) Reviews of NHC−iron complexes: (a) Ingleson, M. J.; Layfield,
́
R. A. Chem. Commun. 2012, 48, 3579. (b) Bezier, D.; Sortais, J.-B.;
Darcel, C. Adv. Synth. Catal. 2013, 355, 19.
REFERENCES
■
(10) Selected examples: (a) Louie, J.; Grubbs, R. H. Chem. Commun.
2000, 1479. (b) McGuinness, D. S.; Gibson, V. C.; Steed, J. W.
Organometallics 2004, 23, 6288. (c) Deng, L.; Holm, R. H. J. Am.
(1) (a) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.;
Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc.
2001, 123, 9222. (b) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 10782. (c) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers,
K. R.; Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.;
Holland, P. L. J. Am. Chem. Soc. 2006, 128, 756. (d) Rose, R. P.; Jones,
C.; Schulten, C.; Aldridge, S.; Stasch, A. Chem. Eur. J. 2008, 14, 8477.
(e) Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558.
(f) Rodriguez, M. M.; Bill, E.; Brennessel, W. W.; Holland, P. L. Science
2011, 334, 780.
́
Chem. Soc. 2008, 130, 9878. (d) Cesar, V.; Lugan, N.; Lavigne, G. J.
Am. Chem. Soc. 2008, 130, 11286. (e) Liu, B.; Xia, Q.; Chen, W.
Angew. Chem., Int. Ed. 2009, 48, 5513. (f) Gao, H.-H.; Yan, C.-H.; Tao,
X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.; Zhang, Y. Organometallics 2010,
29, 4189. (g) Xiang, L.; Xiao, J.; Deng, L. Organometallics 2011, 30,
2018. (h) Zlatogorsgy, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.;
Ingleson, M. J. Organometallics 2011, 30, 4974. (i) Cramer, S. A.;
Jenkins, D. M. J. Am. Chem. Soc. 2011, 133, 19342. (j) Hess, J. L.;
Hsieh, C.; Brothers, S. M.; Hall, M. B.; Darensbourg, M. Y. J. Am.
Chem. Soc. 2011, 133, 20426. (k) Danopoulos, A. A.; Braunstein, P.;
Stylianides, N.; Wesolek, M. Organometallics 2011, 30, 6514.
(l) Meyer, S.; Orben, C. M.; Demeshko, S.; Dechert, S.; Meyer, F.
Organometallics 2011, 30, 6692. (m) Layfield, R. A.; McDouall, J. J.;
Scheer, M.; Schwarzmaier, C.; Tuna, F. Chem. Commun. 2011, 10623.
(n) Wang, X.; Mo, Z.; Xiao, J.; Deng, L. Inorg. Chem. 2013, 52, 59−65.
(2) (a) Karsch, H. H.; Klein, H.-F.; Schmidbaur, H. Chem. Ber. 1977,
110, 2200. (b) Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. J.
Am. Chem. Soc. 1978, 100, 4080. (c) Ittel, S. D.; Tolman, C. A.;
English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1978, 100, 7577.
(d) Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. J. Am. Chem.
Soc. 1979, 101, 1742. (e) Baker, M. V.; Field, L. D. J. Am. Chem. Soc.
1986, 108, 7433. (f) Klein, H.-F.; Camadanli, S.; Beck, R.; Leukel, D.;
Florke, U. Angew. Chem., Int. Ed. 2005, 44, 975.
̈
́
(o) Cesar, V.; Castro, L. C. M.; Dombray, T.; Sortais, J.-B.; Darcel, C.;
(3) (a) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853. (b) Nicolet, Y.; Piras, C.; Legrand, P.;
Hatchikian, C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13.
(c) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12969.
Labat, S.; Miqueu, K.; Sotiropoulos, J.-M.; Brousses, R.; Lugan, N.;
Lavigne, G. Organometallics 2013, 32, 4643−4655. (p) Evans, J.;
Bickelhaupt, F. M.; Layfield, R. A. J. Am. Chem. Soc. 2013, 135,
13338−13341.
̀ ́
(d) Nicolet, Y.; De Lacey, A. L.; Vernede, X .; Fernandez, V. M.;
(11) (a) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129,
7008. (b) Duan, L.; Wang, M.; Li, P.; Na, Y.; Wang, N.; Sun, L. Dalton
Trans. 2007, 1277. (c) Song, L.-C.; Luo, X.; Wang, Y.-Z.; Gai, B.; Hu,
Q.-M. J. Organomet. Chem. 2009, 694, 103.
(12) (a) Lavallo, V.; Grubbs, R. H. Science 2009, 326, 559.
(b) Lavallo, V.; El-Batta, A.; Bertrand, G.; Grubbs, R. H. Angew.
Chem., Int. Ed. 2011, 50, 268.
Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123,
1596. (e) Nicolet, Y.; Cavazza, C.; Fontecilla-Camps, J. C. J. Inorg.
Biochem. 2002, 91, 1. (f) Pandey, A. S.; Harris, T. V.; Giles, L. J.;
Peters, J. W.; Szilagyi, R. K. J. Am. Chem. Soc. 2008, 130, 4533.
(4) Selected examples: (a) Seyferth, D.; Henderson, R. S.; Song, L. C.
Organometallics 1982, 1, 125. (b) Winter, A.; Zsolnai, L.; Huttner, G.
Z. Naturforsch. 1982, 37, 1430. (c) Seyferth, D.; Womack, G. B.;
H
dx.doi.org/10.1021/om401039z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(13) (a) Danopoulos, A. A.; Wright, J. A.; Motherwel, W. B. Chem.
Commun. 2005, 784. (b) Vogel, C.; Heinemann, F. W.; Sutter, J.;
Anthon, C.; Meyer, K. Angew. Chem., Int. Ed. 2008, 47, 2681.
(c) Scepaniak, J. J.; Fulton, M. D.; Bontchev, R. P.; Duesler, E. N.;
Kirk, M.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 10515.
10782. (c) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127,
1913. (d) Thomas, C. M.; Mankad, N. P.; Peters, J. C. J. Am. Chem.
Soc. 2006, 128, 4956. (e) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P.
J. J. Am. Chem. Soc. 2006, 128, 5302. (f) Eckert, N. A.; Vaddadi, S.;
Stoian, S.; Lachicotte, R. J.; Cundari, T. R.; Holland, P. L. Angew.
Chem., Int. Ed. 2006, 45, 6868. (g) Lu, C. C.; Saouma, C. T.; Day, M.
W.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 4. (h) Ni, C.; Fettinger, J.
C.; Long, G. J.; Brynda, M.; Power, P. P. Chem. Commun. 2008, 6045.
(i) Cowley, R. E.; DeYonker, N. J.; Eckert, N. A.; Cundari, T. R.;
DeBeer, S.; Bill, E.; Ottenwaelder, X.; Flaschenriem, C.; Holland, P. L.
Inorg. Chem. 2010, 49, 6172. (j) King, E. R.; Hennessy, E. T.; Betley,
T. A. J. Am. Chem. Soc. 2011, 133, 4917. (k) Moret, M.- E.; Peters, J. C.
Angew. Chem., Int. Ed. 2011, 50, 2063.
(24) Selected examples of iron complexes with bridging imido
ligands: (a) Nichols, P. J.; Fallon, G. D.; Murray, K. S.; West, B. O.
Inorg. Chem. 1988, 27, 2795. (b) Verma, A. K.; Nazif, T. N.; Achim, C.;
Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013. (c) Link, H.; Decker, A.;
Fenske, D. Z. Anorg. Allg. Chem. 2000, 626, 1567. (d) Duncan, J. S.;
Nazif, T. M.; Verma, A. K.; Lee, S. C. Inorg. Chem. 2003, 42, 1211.
(e) Brown, S. D.; Mehn, M. P.; Peters, J. C. J. Am. Chem. Soc. 2005,
127, 13146. (f) Takemoto, S.; Ogura, S.; Yo, H.; Hosokoshi, Y.;
Kamikawa, K.; Matsuzaka, H. Inorg. Chem. 2006, 45, 4871. (g) Duncan,
J. S.; Zdilla, M. J.; Lee, S. C. Inorg. Chem. 2007, 46, 1071. (h) Chen, X.-
D.; Duncan, J. S.; Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 2010, 132,
15884. (i) Zdilla, M. J.; Verma, A. K.; Lee, S. C. Inorg. Chem. 2011, 50,
1551.
̈
(14) (a) Ofele, K.; Kreifer, C. G. Chem. Ber. 1972, 105, 529.
(b) Huttner, G.; Gartzke, W. Chem. Ber. 1972, 105, 2714. (c) Lappert,
M. F.; Pye, P. L. J. Chem. Soc., Dalton Trans. 1977, 2172. (d) Lappert,
M. F.; MacQuitty, J. J.; Pye, P. L. J. Chem. Soc., Dalton Trans. 1981,
1583. (e) Hitchcock, P. B.; Lappert, M. F.; Thomas, S. A.; Thorne, A.
J.; Carty, A. J.; Taylor, N. J. J. Organomet. Chem. 1986, 315, 27.
(f) Warrats, S.; Postigo, L.; Royo, B. Organometallics 2013, 32, 893.
(g) Li, H.; Castro, L. C. M.; Zheng, J.; Roisnel, T.; Dorcet, V.; Sortais,
B.; Darcel, C. Angew. Chem., Int. Ed. 2013, 52, 8045.
(15) (a) Ohki, Y.; Takikawa, Y.; Hatanaka, T.; Tatsumi, K.
Organometallics 2006, 25, 3111. (b) Ohki, Y.; Hatanaka, T.;
Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 17174. (c) Hatanaka, T.;
Ohki, Y.; Tatsumi, K. Chem. Asian J. 2010, 5, 1657. (d) Hatanaka, T.;
Ohki, Y.; Kamachi, T.; Nakayama, T.; Yoshizawa, K.; Katada, M.;
Tatsumi, K. Chem. Asian J. 2012, 7, 1231. (e) Hashimoto, T.; Urban,
S.; Hoshino, R.; Ohki, Y.; Tatsumi, K.; Glorius, F. Organometallics
2012, 31, 4474. (f) Grohmann, C.; Hashimoto, T.; Frolich, R.; Ohki,
̈
Y.; Tatsumi, K.; Glorius, F. Organometallics 2012, 31, 8047.
(g) Hatanaka, T.; Ohki, Y.; Tatsumi, K. Eur. J. Inorg. Chem. 2013,
33, 3966.
(16) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. J. Organometallics
(25) (a) Carty, A. J. Organomet. Chem. Rev. A 1972, 7, 191.
(b) Haiduc, I.; Silaghi-Dumitrescu, I. Coord. Chem. Rev. 1986, 74, 127.
(c) Moore, D. S.; Robinson, S. D. Adv. Inorg. Chem. 1986, 30, 1.
(26) Cowley, R. E.; Bill, E.; Neese, F.; Brennessel, W. W.; Holland, P.
L. Inorg. Chem. 2009, 48, 4828.
(27) The fate of the liberated NHC ligand remains unclear.
(28) Relevant 1,3-dipolar cycloaddition reactions of metal−azides
and metal−acetylides are known: (a) Del Castillo, T. J.; Sarkar, S.;
Abboud, K. A.; Veige, A. S. Dalton Trans. 2011, 40, 8140−8144.
(b) Powers, A. R.; Yang, X.; Del Castillo, T. J.; Ghiviriga, I.; Abboud,
K. A.; Veige, A. S. Dalton Trans. 2013, 42, 14963−14966.
(29) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518−1520.
(30) (a) Arduengo, A. J., III Preparation of I,3-Disubstituted
Imidazolium Salts. U.S. Patent No. 5077414, 1991. (b) Kuhn, N.;
Kratz, T. Synthesis 1993, 561. (c) Arduengo, A. J., III; Krafczyk, R.;
Schmutzler, R. Tetrahedron 1999, 55, 14523.
(31) Bergbreiter, D. E.; Killough, J. M. J. Am. Chem. Soc. 1978, 100,
2126.
2012, 31, 3264.
(17) (a) Saino, N.; Kogure, D.; Okamoto, S. Org. Lett. 2005, 7, 3065.
(b) Bedford, R. B.; Betham, M.; Bruce, D. W.; Danopoulos, A. A.;
Frost, R. M.; Hird, M. J. Org. Chem. 2006, 71, 1104. (c) Hatakeyama,
T.; Nakamura, M. J. Am. Chem. Soc. 2007, 129, 9844. (d) Noda, D.;
Sunada, Y.; Hatakeyama, T.; Nakamura, M.; Nagashima, H. J. Am.
Chem. Soc. 2009, 131, 6078. (e) Hatakeyama, T.; Hashimoto, S.;
Ishizuka, K.; Nakamura, M. J. Am. Chem. Soc. 2009, 131, 11949.
(f) Hatakeyama, T.; Hashimoto, S.; Kondo, Y.; Fujiwara, Y.; Seike, H.;
Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem. Soc.
2010, 132, 10674.
(18) (a) Nieto, I.; Ding, F.; Bontchev, R. P.; Wang, H.; Smith, J. M. J.
Am. Chem. Soc. 2008, 130, 2716. (b) Scepaniak, J. J.; Young, J. A.;
Bontchev, R. P.; Smith, J. M. Angew. Chem., Int. Ed. 2009, 48, 3158.
(c) Smith, J. M.; Long, J. R. Inorg. Chem. 2010, 49, 11223. (d) Zhang,
Q.; Xiang, L.; Deng, L. Organometallics 2012, 31, 4537.
(19) Lee, C. H.; Laitar, D. S.; Mueller, P.; Sadighi, J. P. J. Am. Chem.
Soc. 2007, 129, 13802.
(20) (a) Radonovich, L. J.; Eyring, M. W.; Croshens, T. J.; Klabunde,
K. J. J. Am. Chem. Soc. 1982, 104, 2816. (b) Nametkin, N. S.; Tyurin,
V. D.; Nekhaev, A. I.; Sobolev, Y. P.; Kondrateva, M. G.; Batsanov, A.
S.; Struchkov, Y. T. J. Organomet. Chem. 1983, 243, 323. (c) Zenneck,
U.; Frank, W. Angew. Chem., Int. Ed. 1986, 25, 831. (d) Mandon, D.;
Toupet, L.; Astruc, D. J. Am. Chem. Soc. 1986, 108, 1320. (e) Schulz,
H.; Pritzkow, H.; Siebert, W. Chem. Ber. 1992, 125, 987. (f) Brodt, C.;
Niu, S.; Pritzkow, H.; Stephan, M.; Zenneck, U. J. Organomet. Chem.
1993, 459, 283. (g) Kubo, H.; Hirano, M.; Komiya, S. J. Organomet.
Chem. 1998, 556, 89. (h) Floch, P. L.; Knoch, F.; Kremer, F.; Mathey,
F.; Scholz, J.; Scholz, W.; Thiele, K.-H.; Zenneck, U. Eur. J. Inorg.
Chem. 1998, 119. (i) Wadepohl, H.; Gebert, S.; Pritzkow, H. J.
Organomet. Chem. 2000, 614−615, 158. (j) Donnecke, D.; Wunderle,
̈
J.; Imhof, W. J. Organomet. Chem. 2004, 689, 585. (k) Nakajima, Y.;
Ozawa, F. Organometallics 2012, 31, 2009.
19
(21) Whereas Ni2(μ-η1:η6-IPr)2 is known to bind C6D6 to afford
(IPr)Ni(η6-C6D6), complexes 2a,b do not immediately react with
1
C6D6. The H NMR spectra of 2a,b in C6D6 and THF-d8 are similar
and did not change for a few days at room temperature. The magnetic
properties of 2a,b at higher temperature are yet to be analyzed.
(22) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Sur, S. K. J. Magn.
Reson. 1989, 82, 169.
(23) Selected examples of terminal imido complexes of iron:
(a) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 322. (b) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125,
I
dx.doi.org/10.1021/om401039z | Organometallics XXXX, XXX, XXX−XXX